feat(db): Campaign SQLModel + repo write/read methods

Adds the campaigns table and the BaseRepository / SQLModelRepository
methods that the campaign-clusterer worker (next commit) needs to
populate it. Mirrors the AttackerIdentity layer: schema_version from
day one for federation gossip, soft-merge via merged_into_uuid with a
chain-walking get_campaign_by_uuid, list_campaigns excluding merged-
out rows while list_all_campaigns returns the unfiltered set for the
revoke pass. attacker_identities.campaign_id gets a real FK now that
the target table exists.

decnet/web/db/models/__init__.py

@@ -39,6 +39,10 @@ from .attackers import (
 from .attacker_intel import (
     AttackerIntel,
 )
+from .campaigns import (
+    Campaign,
+    CampaignsResponse,
+)
 from .deploy import (
     DeployIniRequest,
     DeployResponse,